Fe-MAN Challenge

The Fe-MAN challenge invited theoretical chemistry groups to:

• calculate reaction path energetics and/or
• calculate rate constants at 310K

of the protolysis reactions of organoferrate(II) anions, Ar₃Fe⁻ and ROFeR₂⁻ (Ar = Ph, Mes), with fluorinated alcohols ROH (R = CF₃CH₂, CF₂HCH₂) in the gas phase.

An example for such a reaction would be the reaction of Ph₃Fe⁻ with CF₃CH₂OH:

This reaction is provided as a training point. Read more about it here.

Background

In their endeavor to transform synthetic chemistry into more sustainable “green chemistry”, researchers have attempted to use Fe catalysts instead of Pd-based ones for routinely applied reactions such as cross couplings, as Fe is abundant and non toxic for both humans and the environment. However, the study of the involved organoiron species and their microscopic reactivity remains challenging, since Schlenk-type equilibria and solvation effects render the investigations of single organoiron species under well-defined conditions difficult.

The use of electrospray ionization (ESI) mass spectrometry can circumvent these issues. Organometallic ions are prepared under inert-gas conditions, transferred into the gas phase and mass-selected. Subsequently, the microscopic reactivity of the ions of interest is probed by collisional activation and/or ion-molecule reaction studies. In the latter, the ions are stored in quadrupole ion traps where they undergo reactions with a substrate gas. Kinetic measurements then yield rate constants of the reaction at hand. Computational chemistry, in turn, offers structures and energies of the involved reactants, intermediates, transition states and products. Methods of statistical rate theory convert them into theoretical rate constants, to which those from the experiments can be compared quantitatively. The interplay of experiment and theory thus elucidates the reaction mechanism and thereby the microscopic reactivity of the organometallic ion. This methodology was successfully proven for the protolysis of para-substituted trisarylzincate anions by 2,2,2-trifluoroethanol and quantitative agreement between experimental and theoretical rate constants within factors of 2 to 8 was achieved (https://doi.org/10.1021/acs.jpca.1c08964). This translates to about 4 kJ/mol deviations, or in other words chemical accuracy.

We proposed Fe-MAN, which stands for “Ferrates – Microkinetic Assessment of Numerical quantum chemistry” as a first attempt to blind challenge quantum chemical methods in the prediction of activation barriers/rate constants in the gas phase with reference experimental data. The reactions consisted of gas-phase protolysis of organoferrate(II) anions, Ar₃Fe^- and ROFeR₂⁻, by proton donors ROH (Ar = Ph, Mes; R = CF₃CH₂, CF₂HCH₂). These reactions serve as model systems, in which the substrate ROH attacks the nucleophilic carbon moiety of the organoferrate electrophilically and thus, represents the polar reactivity pattern often encountered in the transmetalation step of cross couplings, such as the Sonogashira or Negishi reaction.